Afferent pathways of stressful stimuli: corticotropin release after partial deafferentation of the medial basal hypothalamus

Development and validation of brain target controlled infusion of propofol in mice

Mechanisms through which anesthetics disrupt neuronal activity are incompletely understood. In order to study anesthetic mechanisms in the intact brain, tight control over anesthetic pharmacology in a genetically and neurophysiologically accessible animal model is essential. Here, we developed a pharmacokinetic model that quantitatively describes propofol distribution into and elimination out of the brain. To develop the model, we used jugular venous catheters to infuse propofol in mice and measured propofol concentration in serial timed brain and blood samples using high performance liquid chromatography (HPLC). We then used adaptive fitting procedures to find parameters of a three compartment pharmacokinetic model such that all measurements collected in the blood and in the brain across different infusion schemes are fit by a single model. The purpose of the model was to develop target controlled infusion (TCI) capable of maintaining constant brain propofol concentration at the desired level. We validated the model for two different targeted concentrations in independent cohorts of experiments not used for model fitting. The predictions made by the model were unbiased, and the measured brain concentration was indistinguishable from the targeted concentration. We also verified that at the targeted concentration, state of anesthesia evidenced by slowing of the electroencephalogram and behavioral unresponsiveness was attained. Thus, we developed a useful tool for performing experiments necessitating use of anesthetics and for the investigation of mechanisms of action of propofol in mice.

The anterolateral projections of the medial basal hypothalamus affect sleep

The role of the medial basal hypothalamus (MBH) and the anterior hypothalamus/preoptic area (AH/POA) in sleep regulation was investigated using the Halász knife technique to sever MBH anterior and lateral projections in rats. If both lateral and anterior connections of the MBH were cut, rats spent less time in non-rapid eye movement sleep (NREMS) and rapid eye movement sleep (REMS). In contrast, if the lateral connections remained intact, the duration of NREMS and REMS was normal. The diurnal rhythm of NREMS and REMS was altered in all groups except the sham control group. Changes in NREMS or REMS duration were not detected in a group with pituitary stalk lesions. Water consumption was enhanced in three groups of rats, possibly due to the lesion of vasopressin fibers entering the pituitary. EEG delta power during NREMS and brain temperatures (Tbr) were not affected by the cuts during baseline or after sleep deprivation. In response to 4 h of sleep deprivation, only one group, that with the most anterior-to-posterior cuts, failed to increase its NREMS or REMS time during the recovery sleep. After deprivation, Tbr returned to baseline in most of the treatment groups. Collectively, results indicate that the lateral projections of the MBH are important determinants of duration of NREMS and REMS, while more anterior projections are concerned with the diurnal distribution of sleep. Further, the MBH projections involved in sleep regulation are distinct from those involved in EEG delta activity, water intake, and brain temperature.

Analgesia or hyperalgesia following stress correlates with emotional behavior in rats

Analgesia following exposure to a variety of noxious and non-noxious stressors is well documented and is commonly referred to as stress-induced analgesia. Hyperalgesia following stress has also been reported. The present study shows that a mild stressor (15 min of vibration) produced increased tail-flick latencies (TFL) in some rats, but decreased latencies in other rats. The results of the individual subjects were reproduced in a later session: the rats showing increased TFL on day 1, responded with increased TFL on day 2. Rats showing decreased TFL on day 1 responded with decreased TFL on day 2. Whichever reaction occurred, analgesia or hyperalgesia, this correlated with the animal's behavior during the stress procedure. Analgesia was produced in quiet rats and hyperalgesia in hyperemotional ones. Various peripheral nerve stimulation procedures producing hyperemotional reactions also resulted in lowering of the pain threshold. The results of the present study show behavioral modulation of pain mechanisms.

Differential responsivity of corticosterone and prolactin to stress following lesions of the septum or amygdala: implications for psychoneuroendocrinology

Attempts to use neuroendocrine challenge tests to reflect suspected CNS dysfunction are popular, but frought with problems of interpretation. The dexamethasone suppression test of cortisol is thought to reflect limbic system function. Hormone responses to the triple bolus challenge test have also been reported to reflect limbic system dysfunction, but interpretation is dependent upon the hormone chosen for study. Challenge tests using prolactin have fallen out of favour because the final common pathway for its regulation lies outside the blood brain barrier. Thus, the ability of prolactin to reflect CNS and, specifically limbic system activity has been questioned. The present study undertook to determine corticosterone and prolactin responses in response to a variety of stimuli in rats that had selected limbic system damage following stereotoxically placed electrolic lesions. Surgical groups included: normal, sham-operated, septal lesion, cortico-medial amygdala lesion or baso-lateral amygdala lesion. All CNS damage was verified histologically. The stimuli chosen for neuroendocrine challenge were 3 minutes of exposure to, a novel environment, noise, ether vapour or cold water. These stimuli were chosen for their presumed level of neural input to the hypothalamic-pituitary axis. In the normal group, prolactin, but not corticosterone evidenced a differential response to the four stimuli. Following surgery, corticosterone responses to all stimuli were significantly influenced only in the septal lesion group. There was no interaction between the location of lesion with the type of stimuli used to elicit the corticosterone response. Prolactin, in contrast, evidenced significantly different responses depending on the location of the lesion, the type of stimulus and an interaction of these two variables. This level of differential responsiveness suggests that prolactin may have potential for diagnosis of limbic system dysfunction if appropriate test stimuli are employed to elicit its response.

Suppression of endotoxin-induced corticosterone secretion in rats by H1-antihistamine

Corticosterone (CS) secretion is stimulated in rats by an intraperitoneal injection of bacterial lipopolysaccharide (LPS) or by subjecting the animals to immobilization stress. LPS injection caused a significant increase in the lung histamine level and a sharp reduction in the number of intact peritoneal mast cells. Injection of compound 48/80, a histamine liberator, provoked an increase in the histamine levels of the blood and lung and a decrease in the number of intact peritoneal mast cells with a concomitant increase in CS secretion. Administration of histamine, at a dose of 10 mg/kg, induced a marked increase in CS release. LPS-induced CS secretion was attenuated by pretreatment with an H1-antihistamine, promethazine (PMZ), whereas an H2-antihistamine, metiamide, had no effect. In contrast, PMZ was ineffective on CS release provoked by immobilization stress. These results suggest that LPS-induced CS release is mediated, in part, by histamine released in the peripheral tissues, whereas an immobilization stress-induced increase is not mediated by the amine.

Effects of conditioned fear and environmental novelty on plasma beta-endorphin in the rat

The levels of rat plasma beta-endorphin-like immunoreactivity following a 30 min exposure to (1) an unfamiliar operant chamber, (2) an unfamiliar operant chamber providing a VI-5 second schedule of 3 mA, 1 sec footshocks, or (3) a familiar chamber providing no shocks, but previously paired with unavoidable shocks were compared to control values from animals left undisturbed in their familiar home cages. The shocked group showed ten-fold elevations of beta-endorphin-like immunoreactivity compared to the undisturbed control animals, while conditioned rats showed a smaller two-fold elevation when re-exposed to a chamber in which they had previously been shocked. Two of five rats exposed merely to an unfamiliar chamber showed elevations, but there was no statistically reliable group effect. Such procedures may be useful for controlled and parametric studies of the mobilization of pituitary or brain pools of beta-endorphin under conditions involving merely anticipation of pain rather than the actual activation of ascending nervous pain pathways.

Stress-produced analgesia and morphine-produced analgesia: lack of cross-tolerance

Animals exposed to cold-water swims, rotation, inescapable shocks, abrupt food deprivation and other stressors display temporary analgesia. Since repeated exposures result in adaptation of this analgesia in much the same way that repeated administration of opiates results in tolerance, the possibility of cross-tolerance between cold-water stress-induced and morphine-induced analgesia was investigated. Flinch-jump thresholds were determined in ten experimental groups of six rats each. Three groups showed dose-dependent analgesia following single injections of morphine at 5, 10 and 15 mg/kg, respectively. A fourth group, subjected to a single cold-water swim at 2 degrees C for 3.5 min, displayed analgesia comparable to that produced by 10 mg/kg of morphine. Groups subjected either to 14 daily cold-water swims or to 14 daily morphine injections at 10 mg/kg showed normal thresholds on the 14th day indicating that adaptation and tolerance had developed, respectively. The cross-over groups were exposed to either 13 days of could-water swims followed by morphine or the reverse arrangement. Both groups showed profound analgesia instead of cross-tolerance, suggesting that a non-opiate neural mechanism may mediate stress-induced analgesia.

Failure of noise exposure to modify temporal patterns of plasma cortisol in man

The purpose of this investigation was to assess the modification in the temporal pattern of plasma cortisol levels during exposure to noises of different intensities, frequencies and durations. Plasma cortisol concentrations were measured from 08.00 h to 15.00 h at 10 min intervals in eight subjects on a control day and one or two exposure days. Noise exposures induced no significant increase in plasma cortisol concentration. It is concluded that noise, at "safe" levels for human hearing conservation, when not associated with other potentially noxious stimuli does not cause hyperactivity of the pituitary-adrenocortical system. These results do not tally with those from animal studies where noise is known to activate corticotrophin (ACTH) secretion.

Time course for plasma 11-hydroxycorticosteroid elevation in rats during stress

The time course of plasma 11-hydroxysteroid elevation was studied in two stress situations: regular unsignalled foot shock which produces an intermediate steroid elevation and irregular signalled foot shock with the possibility of escape, which produces an extreme steroid elevation. The initial time course for steroid elevation followed a similar pattern for both treatment groups with the exception that in the irregular signalled group the plasma steroid elevation was more pronounced and there was an indication of biphasic response. The results are discussed in terms of possible inhibitory feedback pathways.